The reaction layer of a fuel cell is formed between an electrolyte film and a diffusion layer, which supports a catalyst which accelerates the electrochemical reaction. For example, in the reaction layer on the air electrode side, protons that pass through the electrolyte film and electrons transmitted to the air electrode are introduced to catalysts, whereby the oxygen ions and protons diffused among the catalysts bond together. That is to say, in order to improve the transmission loss of the oxygen ions, protons, and electrons, there is a need to form the reaction layer such that it has both proton conductivity and electron conductivity. Accordingly, a conventional material used for such a reaction layer is formed by mixing carbon particles (having electron conductivity), on the surface of which the catalysts are supported, and an organic polymer material having proton conductivity such as Nafion (trademark: manufactured by Du Pont, as well as the following trademarks) or the like (see FIG. 1B).
However, such an arrangement in which a material having proton conductivity and another material having electron conductivity are mixed at a macro level has a problem of a difficulty in creating a completely uniform mixture thereof. This leads to a problem in that there are many regions having structures that differ from the structure (three-phase interface) which provides a reactant path, a proton path, and an electron path around the catalyst, leading to a difficulty in smoothly introducing the electrode reaction.
In order to solve this problem, it has been proposed that a mixed conductor having both proton conductivity and electron conductivity at the molecular structure level is employed as a catalyst carrier. With such a mixed conductor, a part having proton conductivity and another part having electron conductivity are positioned extremely closely to each other at a microscopic level, i.e., molecular structure level, thereby reducing such parts having structures that differ from the particular structure (three-phase structure) which provides the reactant path, the proton path, and the electron path around the catalyst. This provides rapid progress of the electrode reaction.
For example, Patent documents 1 through 4 disclose a catalyst supporting carrier employing an organic mixed conductor.
Also, Patent documents 5 through 8 disclose a catalyst supporting carrier employing an inorganic mixed conductor.
[Patent Document 1]
    Japanese Patent Application Publication No. JP-A-2001-202971[Patent Document 2]    Japanese Patent Application Publication No. JP-A-2001-110428[Patent Document 3]    Japanese Patent Application Publication No. JP-A-2003-68321[Patent Document 4]    Japanese Patent Application Publication No. JP-A-2002-536787[Patent Document 5]    Japanese Patent Application Publication No. JP-A-1998-255832[Patent Document 6]    Japanese Patent Application Publication No. JP-A-1999-335165[Patent Document 7]    Japanese Patent Application Publication No. JP-A-2000-251533[Patent Document 8]    Japanese Patent Application Publication No. JP-A-2000-18811